(1) Field of the Invention:
The present invention relates to an air-cooled overhead-valve engine suitable to power lawn mowers and the like.
(2) Description of the Prior Art:
Generally, said engine has been broadly adopted to a structure as shown in FIG. 1. As shown in the FIG. 1, a cooling fan 2 mounted to one end portion of crankshaft 1 is received within a fan casing 3. Said fan casing 3 is extended from the vicinity of crankcase 5 to the vicinity of cylinder block 6 and cylinder head 7. Furthermore, the fan casing 3 is provided with a suction air opening formed coaxially with the crankshaft 1 and an exit section opened towards to the cylinder block 6 and the cylinder head 7, respectively. Also, the fan casing 3 is provided with an intake port 8 for combustion air at the vicinity of the cylinder head 7, and said intake port 8 is connected to a carburetor 10 through an air cleaner 9. The intake port 8 is provided in the direction of air flow f formed by the cooling fan 2, and in details, the intake port 8 is provided in a position, which is difficult to form a flow drift of air flow f.
In the construction in the prior art as shown in the above, the intake port 8 is located at a position of low velocity head, thereby a fuel flow control in the carburetor 10 will be obtained easily, and a suction air to the fan casing 3 and an inflow of dust to the intake port 8 will be inhibited. However, in such prior construction, a cam gear 12 for driving intake and exhaust valves 11, push rods 13 and a rocker arm 14, respectively are arranged on the same side with an intake port 16 with relationship to a cylinder center 0-0 (center line, which is extended in the moving direction of piston 15) as shown in FIG. 2, while an exhaust port 26 is arranged in the opposite side of the intake port 16. Thus, in the arrangement of the cam gear 12 constructed as shown in the above, generally, the cooling fan 2 refers to a centrifuged fan, so that the fan casing 3 is necessary to form it in volute state. In this case, preferably, the engine can be formed compactly by providing the cam gear 12 on a side of a large volute radius of the fan casing 3, but many following disadvantages produce in the configuration of the engine:
That is, in the construction as discussed thereinabove, an inlet pipe 17 is formed to the intake port 16, said pipe 17 is provided with a space in between both push rods 13 as shown by solid line in FIG. 3, or is located in roundabout route as shown by two-dot-line. In either case, it is allowed to complicate the construction. Also, since the intake port 16 is provided with a space in between both push rods 13, a cooling air passage 39 surrounded with a wall 35 of combustion chamber 20, a valve guide boss 36, a rocker arm chamber and a push rod-through portion 38, respectively would be intercepted by the inlet pipe 17, which is defined the intake port 16 and as a result, it is incapable of a substantial flow of cooling air for the cooling of such cylinder head 7 as a most heated surface of the engine. Also, the intake port 16 is provided in such a manner that the intake port 16 is located to make a deviated route from the push rod 13 as shown by the two-dot-line, so that the inlet pipe 17 is necessary to exchange it to an alternative part instead of the integrated cylinder head 7. So alternative is the impact of elevating considerably a manufacturing cost. Also, the intake port 16 extends to long, giving reduction to an intake efficiency of the engine.
Also, in a vertical shaft engine having vertically supported crankshaft, the carburetor 10 is mounted upon a relatively high level, while the air cleaner 9 and fuel tank (not shown in Figure) are mounted upon relatively same level and as a result, the particular structural arrangement of engine is much less a compactness. In addition, the exhaust port 26 is located in the opposite side to the intake port 16, and a spark plug 25 allows approaching nearest to said exhaust port, giving rise high temperature condition, an insufficient cooling of the spark plug results too early ignition in a large negative work of the engine.
Also, Japanese patent application laid-open No. 59-70838 and Japanese utility model application publication No. 1081/84 include therein as references in the prior art.